利用掃瞄穿隧顯微鏡觀測雙原子分子在表面上的反應

Abstract

本論文主要是利用掃瞄穿隧顯微鏡（STM）觀察分子在表面上的作用與反應，其中的課題包括分子結合脫附、表面結構相變化、分子吸附、熱原子動力學以及基底大小對量子點成長的影響。本論文各章節大概編排如下：第一章簡介分子在表面上的吸附以及脫附動力學。第二章描述實驗上的儀器架構、STM操作原理、樣品製備方式以及探針脫附的技巧。而第三章到第五章是展現實驗結果以及討論。第六章為實驗結果的結論以及進一步的討論。 在探討表面結構相變化的部分，氫原子在Si(100)表面上可使表層矽原子鍵結一個氫原子（monohydride）或者鍵結兩個氫原子（dihydride），而這兩種結構的組合可讓Si(100)表面上形成三種穩定的相位：(1×1)、(3×1)以及(2×1)相位，氫原子的覆蓋率分別為2、1.33以及1個表層矽原子密度。本實驗探討(1×1)和(3×1)相位是如何相變到(2×1)相位，以及氫原子由表面熱脫附的動力學機制，做法是加熱擁有(1×1)和(3×1)相位的樣品，使表面上的結構相變至(2×1)相位，再利用掃瞄穿隧顯微鏡觀察表面上的變化。結果顯示氫分子由dihydride區域脫附時（由1×1區域脫附），兩個氫原子來自於相鄰兩個dihydride內側的氫原子。這種脫附方式在(3×1)區域進行前，必須先有dihydride與monohydride位置交換的動作，才能產生相鄰的dihydride。而氫分子由monohydride區域脫附是在樣品溫度較高時，兩個氫原子來自於同一個dimer上的氫原子。 在研究分子吸附動力學部分，首先討論碘分子在乾淨Si(100)表面上的吸附動力學。碘分子覆蓋率較低時，結果顯示兩個碘原子吸附在相鄰同排dimer row同側上的活性鍵的機率最高。當碘的吸附量增加後，最高的覆蓋率只能達到0.92個表層矽原子密度。為了進一步探討碘分子以及氯分子的吸附動力學，我們在覆蓋了氫原子的Si(100)表面上製備出擁有孤立活性鍵的表面，這些活性鍵可以是單一的、成對的、成串的或是長鍊狀的。當碘分子和氯分子與這些活性鍵作用後，我們觀察到兩種分子是以不同的動力學做吸附。對於碘分子的吸附，兩的碘原子的吸附必須同時產生，分子鍵才會斷裂。所以表面上必須存在兩個且鄰近的活性鍵，碘分子才會裂解吸附。對於氯分子而言，兩個氯原子的吸附是可以單獨進行，所以表面上只需要存在單一個活性鍵，即可使吸附反應發生。反應過程往往是一個氯原子先與活性鍵鍵結，而另一個氯原子可以離開表面，也可以與周遭的原子產生新的反應。 本論文利用真實空間以及原子解析度的影像來探討表面化學反應，其中的利用孤立活性鍵來探討分子吸附動力學更是第一次提出，這樣的表面結構提供一個很好的介面來研究氣體與固體反應動力學。

The objective of this thesis focuses on the fundamental issues in surface reactions by using scanning tunneling microscopy (STM). The scientific issues include recombinative desorption, structure phase transition, dissociative adsorption, abstractive adsorption, hot atom motion, and size-dependent quantum-dot growth. The thesis is organized as following: Chapter 1 introduces the common mechanisms of molecules adsorption and desorption on the surface. Chapter 2 describes the experimental setup employed during this research. The STM operating principles, sample preparation procedures, and STM lithography techniques are also explained. The detailed results and discussions of each issue are presented in Chapter 3, 4, and 5. Finally, Chapter 6 summarizes the results and further discussions. For the discussion of structure phase transition on the Si(100) surface, monohydrides dimers (SiH) and dihydride (SiH2) species can form an ordered mixture with (3 × 1), (1 × 1), and (2 × 1) phases. Thermal annealing at 570 K causes both the (3 × 1) and (1 × 1) domains to transform to the (2 × 1) phases. During the hydrogen reduction from 1.33 ML to 1 ML, the recombinative desorption of H2 from dihydride and monohydride species are investigated. Desorption from dihydrides proceeds by recombination of two H atoms coming separately from two adjacent dihydrides. This process is geometrically forbidden for the (3 × 1) surface, but becomes possible with a switch of a dihydride with a neighboring monohydride dimer. Desorption from monohydrides occurs at a higher temperature, and proceeds by recombination of the two H atoms on a given monohydride dimer. In the discussions of molecule adsorption mechanisms, I2 on a prototypical semiconductor surface is observed. Adsorption of I2 on the same side of neighboring dimer is favored than on one dimer with the ratio1 at low coverage. After further I2 exposure, the maximum iodine coverage is 0.92 ML. To distinguish the adsorption geometry for I2, single dangling bond pairs that are fabricated on H/Si(100) surface are used for I2 adsorption. Different adsorption mechanisms of I2 and Cl2 are demonstrated with I2 and Cl2 reaction to dangling bonds in isolation or organized in pairs, clusters, or arrays. Iodine chemisorption is predominantly a pair process involving the bonding of the two I atoms in a I2 molecule onto two neighboring dangling bonds. In sharp contrast, adsorption of Cl2 is dominated by the bonding of just one Cl atom in a Cl2 molecule, with the other Cl atom either leaving the surface or migrating to a nearby area to cause further reactions. This thesis gives the real-space images and detailed atomic processes by in situ studies. The adsorption mechanisms of molecules on various initial active site configurations are first reported. This approach points to opportunities for systematic investigations of the atomistics of gas-surface reactions.